Within the context of the present disclosure, the expression “internal combustion engine” encompasses Otto-cycle engines, diesel engines and also hybrid internal combustion engines, which utilize a hybrid combustion process, and hybrid drives which comprise not only the internal combustion engine but also an electric machine which may be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an enhanced power-to-weight ratio. If the swept volume is reduced, it is thus possible, given the same vehicle boundary conditions, to shift the load collective toward higher loads, at which the specific fuel consumption is lower. In the development of internal combustion engines, it is a basic aim to minimize fuel consumption, wherein the emphasis in the efforts being made is on obtaining good overall efficiency.
Fuel consumption and thus efficiency pose a problem in particular in the case of Otto-cycle engines that is to say in the case of an applied-ignition internal combustion engine. The reason for this lies in the fundamental operating process of the Otto-cycle engine. Load control is generally carried out by means of a throttle flap provided in the intake system. By adjusting the throttle flap, the pressure of the inducted air downstream of the throttle flap can be reduced to a greater or lesser extent. The further the throttle flap is closed, that is to say the more the throttle flap blocks the intake system, the higher the pressure loss of the inducted air across the throttle flap, and the lower the pressure of the inducted air downstream of the throttle flap and upstream of the inlet into the at least three cylinders, that is to say combustion chambers. For a constant combustion chamber volume, it is possible in this way for the air mass, that is to say the quantity, to be set by means of the pressure of the inducted air. This also explains why quantity regulation has proven to be disadvantageous specifically in part-load operation, because low loads need a high degree of throttling and a pressure reduction in the intake system, as a result of which the charge exchange losses increase with decreasing load and increasing throttling. To reduce the described losses, various strategies for dethrottling and applied-ignition internal combustion engine have been developed.
One approach to a solution for dethrottling the Otto-cycle engine is for example, an Otto-cycle engine operating process with direct injection. The direct injection of the fuel is a suitable means for realizing a stratified combustion chamber charge. The direct injection of the fuel into the combustion chamber thus permits quality regulation in the Otto-cycle engine, within certain limits. The mixture formation takes place by the direct injection of the fuel into the cylinders or into the air situated in the cylinders, and not by external mixture formation, in which the fuel is introduced into the inducted air in the intake system.
Another option for optimizing the combustion process of an Otto-cycle engine includes the use of an at least partially variable valve drive. By contrast to conventional valve drives, in which both the lift of the valves and the timing are invariable, these parameters which have an influence on the combustion process, and thus on fuel consumption, can be varied to a greater or lesser extent by means of variable valve drives. If the closing time of the inlet valve and the inlet valve lift can be varied, this alone makes throttling-free and thus loss-free load control possible. The mixture mass which flows into the combustion chamber during the intake process is then controlled not by means of a throttle flap but rather by means of the inlet valve lift and the opening duration of the inlet valve.
For supercharging, use is generally made of an exhaust-gas turbocharger, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is supplied to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained. A charge-air cooling arrangement may additionally be provided, by means of which the compressed charge air is cooled before it enters the cylinders.
The advantage of an exhaust-gas turbocharger for example in relation to a mechanical charger is that no mechanical connection for transmitting power exists or is required between the charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
Problems are encountered in the configuration of the exhaust-gas turbocharging, wherein it is basically sought to obtain a noticeable performance increase at all engine speed ranges. In the case of supercharged internal combustion engines with an exhaust-gas turbocharger, a noticeable torque drop is observed when a certain engine speed is undershot. The effect is undesirable and is thus, also one of the most severe disadvantages of exhaust-gas turbocharging.
The torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. For example, if the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. As a result, the charge pressure ratio likewise decreases in the direction of lower engine speeds, which equates to a torque drop.
Previously, a variety of measures have been used to enhance the torque characteristic of an exhaust gas-turbocharged internal combustion engine, including a small turbine cross section and provision of an exhaust-gas blow-off facility. To this end, the turbine is equipped with a bypass line which branches off from the exhaust-gas discharge system upstream of the turbine and in which a shut-off element is arranged. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a threshold value, a part of the exhaust-gas flow is conducted past the turbine, that is to say is blown off, via a bypass line during the course of the so-called exhaust-gas blow-off. This procedure has the disadvantage that the high-energy blown-off exhaust gas remains unutilized and the supercharging behavior is often insufficient at higher engine speeds.
A turbine having a variable turbine geometry permits a more comprehensive adaptation to the respective operating point of the internal combustion engine by way of adjustment of the turbine geometry or the effective turbine cross section, enabling engine speed-dependent or load-dependent regulation of the turbine geometry to take place to a certain extent.
The torque characteristic of the supercharged internal combustion engine may also be enhanced by means of multiple turbochargers arranged in parallel, for example, by means of multiple turbines of relatively small turbine cross section arranged in parallel. The turbines may be activated successively with increasing exhaust-gas flow rate, similar to sequential supercharging.
The torque characteristic may also be influenced by connecting multiple exhaust-gas turbochargers in series. In one example, connecting two exhaust-gas turbochargers in series, wherein a first exhaust-gas turbocharger serves as a high-pressure stage and a second exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map may be expanded to include both smaller compressor flows and larger compressor flows.
In particular, with the first exhaust-gas turbocharger, which serves as a high-pressure stage, it is possible for the surge threshold to be shifted in the direction of smaller compressor flows; because of which high charge pressure ratios may be obtained even with small compressor flows, which may considerably enhance the torque characteristic in the lower part-load range. This is achieved by using the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which, with increasing exhaust-gas mass flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again downstream of the high-pressure turbine and upstream of the low-pressure turbine, that is to say between the two turbines, wherein a shut-off element is arranged in the bypass line in order to control the exhaust-gas flow conducted past the high-pressure turbine.
The two exhaust-gas turbochargers connected in series further increase the power boost through supercharging. Furthermore, the response behavior of an internal combustion engine with two exhaust-gas turbochargers may be considerably enhanced, particularly in the part-load range compared to a similar internal combustion engine with single-stage supercharging. The reason for this is that the relatively small high-pressure stage is less inert than a relatively large exhaust-gas turbocharger used for single-stage supercharging, because a rotor or impeller of an exhaust-gas turbocharger of smaller dimensions may accelerate and decelerate more quickly.
This also has advantages with regard to particle emissions. In a large single exhaust-gas turbocharger, during acceleration, the required increase in the air mass supplied to the cylinders for the increased fuel flow rate takes place with a delay owing to the inertia of the large impellers. In contrast, with a relatively small high-pressure turbocharger, the charge air is supplied to the engine virtually without a delay, and thus operating states with increased particle emissions are more commonly eliminated.
Exhaust-gas turbocharging in combination with exhaust-gas aftertreatment has proven to be problematic. When using an exhaust-gas turbocharger, it is fundamentally sought to arrange the turbine of the charger as close to the engine, that is to say to the outlet openings of the cylinder, as possible in order thereby to be able to optimally utilize the exhaust-gas enthalpy of the hot exhaust gases, which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature, and to ensure a fast response behavior of the turbocharger. Furthermore, the path of the hot exhaust gases to the different exhaust-gas aftertreatment systems may also be as short as possible such that the exhaust gases are given little time to cool down and the exhaust-gas aftertreatment systems reach their operating temperature or light-off temperature as quickly as possible, in particular after a cold start of the internal combustion engine.
The inventors herein have recognized the above cited potential issues, and propose an engine including a first cylinder group connected to a first turbine through a first exhaust line, a second cylinder group connected to a second turbine through a second exhaust line, the second turbine parallel to the first turbine, a first compressor downstream of a second compressor arranged in series along an intake system, and a connecting line connecting the first exhaust line to the second exhaust line. The connecting line branches off from the first exhaust line upstream of the first turbine and connects to the second exhaust line upstream of the second turbine. A controller may regulate exhaust flow through the above described system responsive to engine operating conditions by regulating the position of valves along the first exhaust line and the second exhaust line, engaging or disengaging the turbochargers, depending on engine operating parameters.
In another example, exhaust flow may be regulated through a system including a first group of cylinders each having a respective first exhaust valve, a second group of cylinders each having a respective first exhaust valve and a respective second exhaust valve, a first exhaust line with a first turbine of a first turbocharger connected to the first group of cylinders through the respective first exhaust valves, a second exhaust line with a second turbine of a second turbocharger connected to the second group of cylinders through the respective second exhaust valves, a first compressor of the first turbocharger arranged along a first intake line and a second compressor of the second turbocharger along a second intake line, wherein the first compressor is parallel to the second compressor, and a controller controlling exhaust flow by actuating the first exhaust valves and the second exhaust valves. The first exhaust valves may each have a larger cross-sectional area than the second exhaust valves. The first group of cylinders may include all cylinders of the engine and the second group of cylinder may include two outer cylinders of the first group of cylinders.
In another example, a supercharged internal combustion engine may include a first low-pressure turbocharger with a first turbine and a first compressor, and a second high-pressure turbocharger with a second turbine and a second compressor, the first turbocharger and second turbocharger arranged in series along an exhaust section and an intake section of the engine, with the second turbine arranged upstream of the first turbine in the exhaust section and the second compressor arranged downstream of the first compressor in the intake system, a first bypass line with a first valve, the first bypass line branching off from a first junction point from the exhaust section between the first turbine and the second turbine and opening into the intake section downstream of the first compressor and upstream of the second compressor, a second bypass line with a second valve, the second bypass line branching off from the exhaust section upstream of the second turbine and opening back into the exhaust section again between the first turbine and the second turbine, a third bypass line with a third valve, the third bypass line connecting the intake system from upstream of the first compressor to upstream of the second compressor, at least one exhaust-gas aftertreatment system along the exhaust section downstream of the first turbine and the second turbine, and a charge-air cooler arranged in the intake system between the first compressor and the second compressor.
These arrangements of the supercharged internal combustion engine with two superchargers arranged in series may generate adequate boost pressure to meet torque demand at different engine operating conditions, including at various engine load and engine speed conditions, thereby increasing supercharger efficiency. Additionally, flowing the exhaust gas, under all operating conditions, through at least one turbine, while bypassing a turbine at least during some conditions, before flowing to a downstream aftertreatment device may enable the aftertreatment device to quickly reach light-off temperature, especially during cold start conditions, while still achieving desired boost.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.